A MID-AIR collision was averted Tuesday night in Bengaluru airspace after two IndiGo aircraft came within close range of each other, triggering their collision alarm systems.

Sources told The Indian Express that the aircraft — 6E779 from Coimbatore to Hyderabad with 162 passengers and 6E6505 from Bengaluru to Kochi with 166 passengers — were less than four miles apart, laterally, when the alarm was triggered.

A senior official at the Directorate General of Civil Aviation (DGCA) told The Indian Express that the aircraft warnings were triggered because the rate of climb of one of the aircraft was on the “higher side”, adding that the vertical separation between the two aircraft was 950 feet. “It falls in Category C airprox (aircraft proximity) and was not a serious one,” the official said.

However, industry insiders suggested that at the closest separation point, the vertical distance between the two planes was anywhere between 200 feet and 475 feet. Based on rules prescribed by the International Civil Aviation Organization (ICAO), the DGCA has laid down classifications of aircraft proximity based on distance between the aircraft as well as their relative positions and speeds. This is divided into four categories — Category A being the most serious one and Category D bearing the least risk.

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“The ATC asked one of the aircraft to climb to 36,000 feet and the other to climb to 28,000 feet. At around 27,000 feet, both the aircraft came within the separation that activated the TCAS (Traffic Collision Avoidance System),” sources said.

The TCAS, which is ATC-independent, monitors airspace around the airplane for other aircraft equipped with similar equipment. The system is mandated by ICAO, and is to be equipped on all aircraft with a maximum take-off mass of over 5,700 kg or authorised to carry more than 19 passengers.

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Confirming the incident, IndiGo said: “The TCAS-Resolution Advisory system was triggered on two of our aircraft operating Coimbatore-Hyderabad and Bangalore-Cochin routes respectively on July 10, 2018. Following normal procedure, this has been reported to the regulator.”

What is a midair collision?

A near midair collision is defined as an incident associated with the operation of an aircraft in which a possibility of collision occurs as a result of proximity of less than 500 feet to another aircraft, or a report is received from a pilot or a flight crew member stating that a collision hazard existed between two or more aircraft.